I don't know the process Rob uses in making the metal Reos and the wooden reos, so why is it that the wooden reos have side buttons and the metal ones do not?
I never thought too much about this, but I think it has a lot to do with the fact the body of the metal ones is the circuit. The wood ones are ... well, wood. They are not conductive. So, a wire is necessary to have a circuit. I suppose he might have decided to put the button just about anywhere, but the side was just plain appropriate. If it were on top, the connectors would be right on top of the battery.